One common type of photodiode, which is illustrated in FIG. 3 of U.S. Pat. No. 4,127,932, in which there is a common assignee with the present application and one common inventor, is a p-i-n type semiconductor photodiode with a p+ substrate which acts as the anode, a p- type epitaxial layer, and an n+ type surface region which acts as the cathode. In order to achieve high-speed operation with short response time, it is desirable to have a relatively thick epitaxial layer and a large enough reverse bias voltage to deplete the epitaxial layer such that essentially all light-induced electrons result in electron-hole pairs in the epitaxial layer, with the electrons relatively quickly exiting the epitaxial layer into the cathode, and the holes relatively quickly reaching the anode exiting the epitaxial layer into the substrate. Any electrons which are induced as a result of light which reaches the substrate must first relatively slowly diffuse into the epitaxial layer before they move relatively quickly to the cathode. This type of photodiode is typically operated with the magnitude of the reverse bias being sufficient to essentially completely deplete the epitaxial layer. As the thickness of the epitaxial layer is increased so as to enhance response time, the magnitude of the needed applied reverse bias voltage increases as the square of the thickness of the epitaxial layer. This requires in many applications a reverse bias potential of typically 30 volts or more for a p-i-n diode. Many of today's photodiodes operate with solid-state integrated circuits that use a 5 volt power supply. The requirement of a separate 30 volt supply places additional expense on the overall system, increases potential failure of lower voltage integrated circuits used with the photodiode, and thus is undesirable. Another problem with photodiodes is that the reverse bias leakage current with no light applied, the "dark current", limits operating ranges.
It is desirable to have a silicon p-i-n photodiode which operates with a supply voltage of typically 5 volts or less, can achieve high-speed operation with relatively short response time, and has a relatively small dark current. It is also desirable to have a silicon avalanche photodiode which can be operated in the 100 volt range, can achieve high-speed operation with short response time, and has a relatively small dark current.